Traditional methods of driving compressors using conventional electric motors include not only direct drive connections, but also intermediate couplings, belts, pulleys and gears. In the simplest known form there is direct drive with the motor rotor joined directly onto the input shaft of a compressor. Such a known compressor and motor assembly is shown in FIG. 1. It includes a rotary screw air compressor 1 having bearings 7, and an extended male input shaft 2 which is connected to the rotor 3 of the motor 4. Even in this simplest form the shear weight of the rotor requires the rotor 3 and input shaft 2 to be supported on additional bearings 5.
Various types of motors are known to be used to drive compressors. For example, fixed speed drive motors known to be used to drive rotary screw air compressors, whether flooded or oil free, have involved induction motors linked to various mechanisms of air flow rate control, such as automatic start and stop and more sophisticated inlet controls which limit flow rate in response to variation in system pressure.
Variable speed drive motors are also known for use with compressors such as rotary screw air compressors. These include mechanical speed variation motors, variable speed induction motors (VFD), and variable speed switched reluctance motors (SRD).
Although motors with mechanical speed variation have been tried in limited air compressor applications, these suffer disadvantages of underlying complexity, questionable reliability and overall poor efficiency.
Variable speed induction motors driven by variable frequency inverters are used more widely. The disadvantages are that there are losses in efficiency due to the electronics involved and limitations imposed by the induction motor itself. Although enhanced in most cases with improved insulation, the induction motor can still be nevertheless a compromise from the reliability and efficiency standpoint.
In addition, variable frequency induction motors even if modified to withstand the rigours of variable frequency inverter drive are large and intrinsically heavy units. All the compressor products on the market today using VFD drive systems are a development of existing compressor packaging technologies due to the physical constraints imposed by the use of the induction motors. In all these cases the motors which drive the compressors are usually without gears but with adapter housings, couplings, etc. due to the sizes and masses of the motors involved. Additionally, due to the method of construction and the size of the motor windings, rotor assembly, etc., conventional motor construction involving heavy castings, bearings, end shields, couplings, adapter housings and other relatively expensive components are required.
Compressors are also known involving switch reluctance main drive motors and suitably modified variable frequency inverter drives (SRD), however these also have similar disadvantages.
As a separate matter, hybrid permanent magnet motors are, in themselves, known. In these, magnetic flux to drive the rotor is produced by both permanent magnets and current flow in electromagnetic coils. Examples are described in, for example, U.S. patents U.S. Pat. No. 4,079,278 and U.S. Pat. No. 4,830,412, United Kingdom patent application GB-A-2291274 and European patent EP-A-0780954.